Instrument for holding and inserting a bone anchor

ABSTRACT

An instrument for holding and inserting a bone anchor into a bone is provided, where the bone anchor includes a shank for anchoring in the bone and a head. The instrument includes a holding member for holding the head of the bone anchor and has a plurality of arms configured to at least partially encompass the head, where the arms include an inner surface forming a seat for the head. The instrument also includes a drive shaft for engaging the head of the bone anchor and defining a longitudinal axis of the instrument. The instrument further includes a displacement member configured to act on the holding member. The holding member can assume a first configuration in which it is configured to permit the head to enter the seat and a second configuration in which it is configured to hold the head in the seat.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/645,307, filed Mar. 11, 2015, which claims the benefit of U.S.Provisional Patent Application Ser. No. 61/953,548, filed on Mar. 14,2014, the contents of which are hereby incorporated by reference intheir entirety, and claims priority to European Patent Application EP14160107.0, filed on Mar. 14, 2014, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND Field of the Invention

The present disclosure relates to an instrument for holding andinserting a bone anchor into a bone. The instrument includes a holdingmember including at least two arms that are configured to hold a head ofthe bone anchor between them. The instrument also includes adisplacement member that acts on the holding member such that theholding member can assume a first configuration in which the holdingmember is configured to receive the head, and a second configuration inwhich the holding member holds an inserted head. Further, the instrumentincludes a drive shaft that is configured to engage the head of the boneanchor. The instrument may be used with a polyaxial bone anchoringdevice of a bottom loading type that has a bone anchor and a receivingpart to receive a rod to connect the rod to the bone anchor, where thebone anchor is first inserted into the bone using the instrument and thereceiving part is mounted in situ onto the bone anchor thereafter.

Description of the Related Art

An instrument for holding and inserting of bone screws, in particular ofpedicle screws, is known from DE 20 2005 012 781 U1. The instrumentincludes a holding member with at least two gripping arms for a bonescrew and a displacement member that is movable relative to the grippingarms. By means of the displacement member, the gripping arms can assumea configuration which allows the insertion of the bone anchor and aconfiguration in which the bone anchor is held between the arms. Thegripping arms have projections that engage recesses on the bone screw toallow the screwing-in of the bone screw via the gripping arms.

US 2012/0296171 A1 describes an inserter for a bone anchor including adriver for engaging the head of a bone screw and a receiver member witha spherical inner surface portion that becomes engaged with the headand/or the neck of the bone anchor.

SUMMARY

Embodiments of the invention provide an instrument for holding andinserting a bone anchor that provides a safe and robust connectionbetween the bone anchor and the instrument and an automatic alignment ofthe bone anchor and the instrument.

The instrument includes a holding member with a seat for a head of abone anchor. The instrument also includes a displacement member foracting on the holding member such that the holding member can assume afirst configuration in which the holding member permits the head toenter or be inserted into the seat, and a second configuration in whichthe holding member holds the inserted head. The instrument further has adrive shaft configured to engage the head of the bone anchor. A centerof the seat of the holding member can be offset from a center of thehead such that the head is pressed against an abutment surface of thedrive shaft when the head is inserted into the holding member. Thereby,play between an engagement portion of the drive shaft and the head iseliminated. This allows a precise insertion of the bone anchor into abone without experiencing play between the drive shaft and the boneanchor. Also, by means of this design, the connection between theinstrument and the bone anchor is robust.

The head of the bone anchor may be held in the seat in a form-fit orpositive-fit manner. Furthermore, the drive shaft of the instrument mayinclude an engagement portion that engages a corresponding engagementportion of the head of the bone anchor in a form-fit manner. Thereby,the bone anchor can be automatically aligned correctly with the driveshaft.

The size and position of the seat may be designed such that a lower endof the holding member acts as an abutment for a bone surface. Hence,when the bone anchor is inserted into a bone with the instrument, thefree lower end of the holding member can abut against the bone surfaceto indicate that a correct insertion of the bone anchor has been reachedto allow for easier in situ mounting of a receiving part to the boneanchor thereafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the description of embodiments by means of the accompanyingdrawings. In the drawings:

FIG. 1 shows an exploded perspective view of an instrument according toan embodiment of the invention;

FIG. 2 shows a perspective view of the instrument of FIG. 1 in anassembled state in a first configuration where the instrument isconfigured to receive a head of a bone anchor;

FIG. 3 shows a perspective view of the instrument of FIGS. 1 and 2 in asecond configuration where the head of the bone anchor is received andclamped by the instrument;

FIG. 4 shows a cross-sectional view of a portion of the instrument ofFIGS. 1 to 3, the cross-section taken in a plane containing alongitudinal axis of the instrument;

FIG. 5 shows an enlarged cross-sectional view of an upper portion of theinstrument of FIG. 4;

FIG. 6 shows an enlarged cross-sectional view of a lower portion of theinstrument of FIG. 4;

FIG. 7 shows a perspective view of a drive shaft of the instrument ofFIGS. 1 to 6;

FIG. 8 shows enlarged views of two portions of the drive shaft of FIG.7;

FIG. 9 shows a perspective view from a front end of a holding member ofthe instrument of FIGS. 1 to 6;

FIG. 10 shows a perspective view from a rear end of the holding membershown in FIG. 9;

FIG. 11 shows a top view of the rear end of the holding member of FIGS.9 and 10;

FIG. 12 shows a cross-sectional view of the holding member of FIGS. 9 to11, the cross-section taken along line A-A in FIG. 11.

FIG. 13 shows a perspective view from a rear end of a displacementmember of the instrument of FIGS. 1 to 6;

FIG. 14 shows a perspective view from a front end of the displacementmember of FIG. 13;

FIG. 15 shows a top view of the rear end of the displacement member ofFIGS. 13 and 14;

FIG. 16 shows a cross-sectional view of the displacement member of FIGS.13 to 15, the cross-section taken along line B-B in FIG. 15;

FIG. 17 shows a perspective view from a rear end of an actuator of theinstrument according to FIGS. 1 to 6;

FIG. 18 shows a perspective view from a front end of the actuator ofFIG. 17;

FIG. 19 shows a top view from of the rear end of the actuator of FIGS.17 and 18;

FIG. 20 shows a cross-sectional view of the actuator of FIGS. 17 to 19,the cross-section taken along line C-C in FIG. 19;

FIG. 21 shows a cross-sectional view of a portion of the instrument ofFIGS. 1 to 6 in a first step of inserting a head of a bone anchor whenthe instrument is in the first configuration;

FIG. 22 shows a cross-sectional view of a portion of the instrument ofFIGS. 1 to 6 in the first configuration with the head of the bone anchorinserted therein;

FIG. 23a shows a cross-sectional view of the instrument of FIGS. 1 to 6in the second configuration with the head of the bone anchor insertedtherein;

FIG. 23b shows an enlarged cross-sectional view of a detail of FIG. 23a; and

FIGS. 24 to 26 show schematic enlarged cross-sectional views of a lowerportion of various modified embodiments of the instrument with a head ofa bone anchor inserted therein.

DETAILED DESCRIPTION

Referring to FIGS. 1 to 3, an instrument for holding and inserting abone anchor into a bone according to an embodiment of the inventionincludes a drive shaft 1 that is configured to engage a head portion ofthe bone anchor and to transmit torque to the bone anchor. Theinstrument also includes a holding member 2 for receiving and holdingthe head of the bone anchor and a displacement member 3 that isconfigured to act on the holding member 2. The holding member can assumea first configuration as depicted in FIG. 2, where the head of the boneanchor can be inserted into the holding member, and a secondconfiguration as depicted in FIG. 3, where the head of the bone anchoris firmly held in the holding member 2. The instrument further includesan actuator 4 that cooperates with the drive shaft 1 and thedisplacement member 3 to move the displacement member 3 into first andsecond positions corresponding to the first and second configurations ofthe holding member 2. The instrument also includes a handle 5 that isconnectable to the drive shaft 1.

Referring to FIGS. 2 and 3, a bone anchor 100 in this embodimentincludes a threaded shank 101 and a head 102 at one end of the threadedshank 101. The head 102 may have a surface portion that isspherically-shaped and a substantially flat free end surface 103. Thespherical surface portion may include a region spanning a greatestdiameter of the sphere. The bone anchor may be used in polyaxial boneanchoring devices that can be used for stabilization of the spinalcolumn by means of stabilizing rods. The polyaxial bone anchoringdevices may have a receiving part (not shown) that is configured topivotably receive the bone anchor 100 and to receive the spinalstabilization rod (not shown) to couple the rod to the bone anchor. Theinstrument may be used with bottom loading type polyaxial bone anchoringdevices where the head 102 of the bone anchor 100 is inserted into thereceiving part from a bottom end thereof (i.e., from an end of thereceiving part that faces the bone surface).

With reference to FIGS. 4 to 8, the drive shaft 1 may be a monolithicsubstantially rod-shaped member that has a first end or distal end 11and an opposite second end or proximal end 12. At the first end 11, anengagement projection 13 is provided that is configured to engage acorresponding recess in the head 102 of the bone anchor 100 to transmittorque to the head 102. In the embodiment shown, the engagementprojection 13 has a TORX™-shape. However, any other shape is possiblethat allows connecting the engagement projection 13 to the head 102 ofthe bone anchor 100 in a form-fit or positive-fit manner. Following theengagement projection 13, the drive shaft 1 includes a first shaftportion 14 with a first diameter that is greater than the diameter ofthe engagement projection 13. At a front end of the first shaft portion14 nearest the engagement projection 13, an abutment surface 14 a isformed that extends circumferentially around the engagement projection13 and that is substantially flat.

At a side of the first shaft portion 14 opposite to the engagementprojection 13, the first shaft portion 14 has a threaded section 15 withan outer thread that cooperates with a corresponding inner thread of theholding member 2 and that allows the holding member 2 to be mounted ontothe first shaft portion 14. At a distance from the threaded section 15,a section 16 with a larger outer diameter than the threaded section 15is provided, which forms an annular abutment surface 16 a facing towardsthe first end 11 of the drive shaft 1. The abutment surface 16 afunctions as a stop for the holding member 2 when the holding member ismounted to the first shaft portion 14. An intermediate portion 16 b witha slightly smaller diameter than the section 16 may be present toimprove the stability and/or the alignment of the connection between theholding member 2 and the drive shaft 1.

Between the second end 12 and the larger diameter section 16, there is asecond shaft portion 17 that includes a portion 18 with a greater outerdiameter that is located approximately at a middle region of the secondshaft portion 17 between the second end 12 and the section 16. Theportion 18 is threaded and cooperates with the actuator 4. The thread ofthe portion 18 may be a fine pitch thread and more particularly, may bea multi-start fine pitch thread, such as a double-start fine pitchthread. This makes it possible to obtain a greater displacement of thedisplacement member 3 and the actuator 4 in an axial direction of theactuator 4, for example, compared to using a single-start thread, whenthe actuator 4 is screwed back and forth along the threaded portion 18.Hence, the displacement of the displacement member 3 can be achievedwith less turns or in shorter time. An end section 19 of the drive shaft1 adjacent to the second end 12 has one or more longitudinal flats 19 ato provide a form-fit connection to the handle 5, such that the handle 5cannot rotate relative to the drive shaft 1 once the handle 5 is mountedto the drive shaft 1. An annular groove 19 b at the end section 19 maybe provided that cooperates with a corresponding projection in thehandle 5 to prevent inadvertent separation of the handle 5 from thedrive shaft 1.

A longitudinal axis L of the instrument is defined by the cylindricaldrive shaft 1.

Next, the holding member 2 will be explained with reference to FIGS. 9to 12. The holding member 2 is sleeve-shaped and has a first end ordistal end 21 and an opposite second end or proximal end 22. At adistance from the second end 22, an internally threaded section 23 isprovided with a slightly smaller inner diameter than that of the holdingmember 2 at the second end 22. The internally threaded section 23cooperates with the threaded section 15 of the drive shaft 1. A shoulder23 a is formed by the reduction of the inner diameter of the holdingmember 2 that faces towards the second end 22 and that cooperates withthe intermediate portion 16 b of the drive shaft 1. The holding member 2includes a slot 24 that is open to the first end 21 and that extendstransversely through the holding member 2 in a plane including thelongitudinal axis L such that two resilient arms 25 a, 25 b are formedby the slot 24. The slot 24 widens into a substantially elongate opening24 a that is closed near the second end 22. The opening 24 a providesgreater flexibility to the arms 25 a, 25 b.

At a distance from the first end 21, there is an inner sphericalsegment-shaped section that provides a seat 26 for the head 102 of thebone anchor 100. A radius of the inner spherical segment-shaped section26 substantially matches the shape of an outer surface portion of thehead 102. As can be seen in detail in FIG. 23b , a region with a largestdiameter of the sphere defined by the spherical-segment shaped section26 is between an upper edge 26 a and a lower edge 26 b of the seat 26.Hence, when the head 102 is received in the seat 26 and the arms 25 a,25 b are pressed together, the head 102 is held in a form-fit manner inthe seat 26. Adjacent to the seat 26, a cylindrical section 27 isprovided with an inner diameter that is smaller than an inner diameterof the lower edge 26 b of the seat 26 (see also FIG. 23b ). Between thecylindrical section 27 and the first end 21, there is a tapered section28 that tapers and narrows towards the seat 26. When the head 102 isreceived in the seat 26 and the arms 25 a, 25 b are pressed together,the head 102 is held in the holding member 2 such that the head 102cannot be removed through the first end 21 of the holding member 2.

Referring more in detail to FIGS. 6 and 23 b, when the head 102 is heldin the seat 26, the center of the sphere C₁ of the seat 26 is offset inan axial direction from the center of the sphere C₂ of the head 102.More specifically, in this second configuration of the holding member 2,an axial distance d₁ from the center of the sphere of the seat 26 to theabutment surface 14 a of the drive shaft 1 is smaller than an axialdistance d₂ from the center of the sphere C₂ of the head 102 to theabutment surface 14 a. By means of this, when the head 102 is held inthe seat 26, the head 102 experiences a force that tends to move thehead 102 towards the abutment surface 14 a.

The dimensions of the lower portion of the holding member 2, thatincludes the seat 26, the cylindrical portion 27 and the tapered portion28, may, for example, correspond substantially to a receiving part of apolyaxial bone anchoring device or may be slightly larger in an axialdirection. Therefore, when the head 102 is held in the holding member 2and inserted into the bone, the first end 21 of the holding member 2forms an abutment for the bone surface such that the bone anchor 100 isinhibited from further insertion into the bone. At this position, thehead 102 protrudes above the bone surface to such an extent that thereceiving part can be easily mounted thereon.

A length of the holding member 2 is such that when the holding member 2is screwed onto the threaded section 15 of the drive shaft 1 until thesecond end 22 of the holding member 2 abuts against the abutment surface16 a, the first end 21 of the holding member 2 extends beyond the firstend 11 of the drive shaft 1 by a distance. This distance is such thatthe offset between the centers of the spheres C₁ and C₂ of the seat 26and the head 102, respectively, causes the free end surface 103 of thehead 102 to be pressed against the abutment surface 14 a of the driveshaft 1. An inner diameter of the holding member 2 is such that thefirst portion 14 of the drive shaft 1 can pass therethrough and isguided therein.

Furthermore, as depicted in particular in FIG. 12, an outer surfaceportion 29 of the holding member 2 adjacent to the first end 21 of theholding member 2 is tapered such that the outer surface portion 29narrows towards the second end 22. Hence, the holding member 2 mayresemble and function like a collet chuck.

The displacement member 3 will be described with reference to FIGS. 4 to6 and 13 to 16. The displacement member 3 is formed as a substantiallycylindrical sleeve with a first end or distal end 31 and an oppositesecond end or proximal end 32. An inner diameter of the displacementmember 3 is such that, as can be seen in FIGS. 4 to 6, the holdingmember 2 can extend through the displacement member 3. Adjacent to thefirst end 31, an inner surface section 33 of the displacement member 3is slightly tapered such that the displacement member 3 widens towardsthe first end 31. In the direction towards the second end 32, theslightly tapered section 33 is followed by a second slightly taperedsection 34 that tapers at an angle that is smaller than an angle of thetaper of the first tapered section 33. The remaining portions of thedisplacement member 3 have a cylindrical inner surface 35. The functionof the slightly tapered surface sections 34 and 33 are to cooperate withthe tapered outer surface portion 29 of the holding member 2 when thedisplacement member 3 is moved from a first position in the firstconfiguration to a second position in the second configuration. Duringthis displacement, the tapered inner surface sections 34 and 33 slidealong the tapered outer surface portion 29, thereby generating anincreasing pressure onto the arms 25 a, 25 b that presses the armstogether to clamp an inserted head 102.

The second end 32 of the displacement member 3 has a free annular endsurface 32 a that acts as an abutment for the actuator 4 as describedbelow. From the end surface 32 a, a short circular rim 32 b with asmaller inner diameter than the annular end surface 32 a protrudesupward away from other portions of the displacement member 3. Moreover,a flap-like extension or flap 36 projects above the rim 32 b. The flap36 has a substantially cylindrical inner surface, a substantiallycylindrical outer surface, and a trapezoidal contour when viewed fromthe side. A width of the flap 36 adjacent to the rim 32 b is larger thana width of a free end of the flap 36. Furthermore, at the free end, theflap 36 includes an outwardly extending circumferential projection 37that cooperates with a portion of the actuator 4, to be described below.The height of the flap 36 between the end surface 32 a and the outwardprojection 37 corresponds substantially to the height of an innerportion of the actuator 4 in an axial direction. The flap 36 has thefunction of a follower that allows rotation between the displacementmember 3 and the actuator 4, but prevents axial movement of thedisplacement member 3 relative to the actuator 4. Therefore, axialadvancement of the actuator 4 results in a corresponding axialadvancement of the displacement member 3.

The displacement member 3 may further have a plurality of elongatedopenings 38 that extend completely through a cylindrical wall portion ofthe displacement member 3. The openings 38 facilitate the cleaning ofthe parts of the instrument.

A total axial length of the displacement member 3 is such that in afirst position of the actuator 4 that corresponds to the firstconfiguration, the first end 31 of the displacement member 3 has adistance from the first end 21 of the holding member 2 that allows thefirst end 21 of the holding member 2 to spread outward from the firstend 31 of the displacement member 3 to facilitate insertion of the head102. Furthermore, the axial length of the displacement member 3 is suchthat in a second position of the actuator 4 that corresponds to thesecond configuration, the tapered inner sections 33, 34 cooperate withthe tapered outer surface portion 29 of the holding member to clamp theinserted head 102 in the holding member 2.

The actuator 4 will be described with reference to FIGS. 4 to 6 and 17to 20. The actuator 4 includes a first end or distal end 41 and anopposite second or proximal end 42. At a small distance from the secondend 42, a bore 43 with an internal thread is provided that cooperateswith the external thread of the portion 18 of the drive shaft 1. In someembodiments, the internal thread may be a multi-start fine thread.Adjacent to the threaded bore 43, there is an inner substantiallycylindrical section 44 that has a diameter that is greater than an outerdiameter of the threaded portion 18 of the drive shaft 1 such that thethreaded portion 18 of the drive shaft 1 can extend therethrough.Between the cylindrical inner section 44 and the first end 41, a secondcylindrical section 45 is provided with a smaller diameter than thecylindrical section 44, such that an annular shoulder 45 a is providedwithin the actuator 4. An inner diameter of the cylindrical section 45is such that the second shaft portion 17 of the drive shaft 1 and theflap 36 of the displacement member 3 can extend therethrough, as can beseen in particular in FIG. 5. A distance between the shoulder 45 a andthe first end 41 is such that the flap 36 can abut against the shoulder45 a with a lower side of the circumferential outward projection 37 andthe first end 41 of the actuator can abut against the free end surface32 a of the displacement member 3. Hence, the displacement member 3 isconfigured to be connected to the actuator 4 via the flap 36 such thatthe actuator 4 can rotate with respect to the displacement member 3, butthe displacement member 3 is fixed against displacement in an axialdirection with respect to the actuator 4.

The actuator 4 further includes an upper substantially cylindrical outersection 46 adjacent to the second end 42 with a plurality of flats 46 athat facilitate gripping. The flats 46 a are depicted as beingoval-shaped, but can have any other shape. Any number of flats 46 a canbe implemented. In other embodiments, instead of the flats 46 a, othermeans can be used to facilitate gripping, such as, for example,longitudinal ripples or grooves or a roughened surface.

The actuator further includes a second outer cylindrical section 47adjacent to the first end 41, with an outer diameter that substantiallycorresponds to an outer diameter of the displacement member 3, such thatthe outer diameter of the actuator 4 can be flush with the outerdiameter of the displacement member 3 as depicted in FIGS. 3 to 5.

Referring further to FIG. 1, the handle 5 includes a coaxial bore 51that has an internal flat portion that cooperates with the flat 19 a ofthe drive shaft, such that the handle 5 can be mounted onto the rear endportion 19 of the drive shaft 1 in a rotationally fixed manner. The bore51 of the handle 5 may have an internal annular projection that engagesthe groove 19 b of the drive shaft 1, such that the handle 5 may besecured (e.g., clicked) onto the drive shaft 1.

All parts of the instrument that may come into contact with tissue orbone may be made of a body-compatible material, such as, for example,stainless steel.

The instrument is easy to assemble and disassemble. Therefore, theinstrument can be easily cleaned after use. The instrument may beassembled as follows. First, the holding member 2 is screwed onto thedrive shaft 1 such that the first portion 14 of the drive shaft 1extends through the holding member 2. The position of the holding member2 on the drive shaft is limited in an axial direction by the abutment ofthe second end 22 of the holding member against the abutment surface 16a. Then, the displacement member 3 is connected to the actuator 4 suchthat the flap 36 is inserted into the actuator 4 until the projection 37of the flap 36 rests on the shoulder 45 a of the actuator 4 and thefirst end 41 of the actuator 4 rests on the free end surface 32 a of thedisplacement member 3. Then, the pre-assembled displacement member 3 andactuator 4 are placed onto the drive shaft 1 from the second end 12 ofthe drive shaft 1, such that the internal threaded bore 43 of theactuator is screwed onto the externally threaded portion 18 of the driveshaft 1. Finally, the handle 5 is secured (e.g., clicked) onto the rearend section 19 of the drive shaft 1.

The use of the instrument with a bone anchor will be described withreference to FIGS. 21 to 23 b. In the first configuration, the actuator4 is in a first position closer to the second end 12 of the drive shaft1 than it is in the second configuration. As shown in FIG. 2, to reachthe first configuration, which is an “open configuration,” the actuator4 is rotated by a user's hands, for example in a counter clockwisedirection. The displacement member 3 follows this movement and assumes aretracted position relative to the holding member 2 (e.g., the first end31 of the displacement member 3 is retracted relative to the first end21 of the holding member 2). The arms 25 a, 25 b of the holding member 2project out of the displacement member 3 to such an extent that the arms25 a, 25 b are configured to be resiliently spread apart from eachother. As depicted in FIG. 21, in the first configuration, the head 102pushes the two arms 25 a, 25 b away from each other until the head 102snaps into the seat 26. During the insertion, the engagement projection13 of the drive shaft enters into a corresponding engagement recess 104provided at the free end surface 103 of the head 102. The engagementprojection 13 can be inserted into the engagement recess 104 until thefree end surface 103 of the head 102 abuts against the abutment surface14 a of the drive shaft 1. During this insertion step, the shank 101 isaligned with the drive shaft 1. The head 102 is held in a form-fitmanner in the axial direction and also in the seat 26 such that the head102 cannot be removed easily.

By rotating the actuator 4, for example, in a clockwise direction, thedisplacement member 3 is moved towards the first end 21 of the holdingmember 2 as depicted in FIG. 3. When the displacement member 3 movestowards the first end 21 of the holding member 2, the tapered innersurfaces 33, 34 of the displacement member 3 comes into contact with theouter tapered surface portion 29 of the holding member 2 as depicted inFIG. 23a . Thereby, the two arms 25 a, 25 b are pressed together.Simultaneously, the pressure is transmitted to the head 102, as depictedthrough the horizontal arrow F_(r) in FIG. 23b . Because the center ofthe sphere C₁ of the spherical seat 26 is offset with respect to thecenter of the sphere C₂ of the head 102 in a direction towards theabutment surface 14 a of the drive shaft 1, a force F_(μ) acts onto thehead 102, with a component directed in an axial direction towards theabutment surface 14 a. As a result thereof, the head 102 is firmlypressed onto the abutment surface 14 a. Possible play between theengagement projection 13 and the engagement recess 104 is eliminated.Thereby, the surgeon can insert or implant the bone anchor 100 withoutany play.

In the above second configuration, which is a “closed configuration,”the head 102 is firmly held in the holding member 2. In thisconfiguration, the head 102 can be inserted into the bone. The first end21 of the holding member 2 serves as an abutment for the bone thatlimits the insertion of the bone anchor 100 into the bone and ensuresthat a sufficient portion of the bone anchor projects over the bonesurface, such that a receiving part can be later mounted onto the head102.

An embodiment of a system or a kit can include the above describedinstrument and a bone anchor 100 having a head 102, wherein the seat 26and the dimensions of the holding member 2 are adapted to the dimensionof the head 102.

Various modifications of the above described embodiments are alsopossible.

For example, referring to FIGS. 24 to 26, the seat 26 for the head 102in the holding member 2 can have various different shapes. Parts andportions that are identical to the previously described embodiment aremarked with the same reference signs, and the descriptions thereof willnot be repeated. As depicted in FIG. 24, the holding member 2′ may havea seat 26′ that has a spherical segment shape 26 a′ adjacent to thelower cylindrical portion 27. The seat 26′ also includes a slightlyconically widening portion 26 b′ that widens in a direction towards thesecond or proximal end 22 of the holding member 2′. As in the previousembodiment, the size of the seat 26′ is such that when the arms 25 a, 25b are compressed, the head 102 experiences a force that presses the head102 towards the abutment surface 14 a.

In the embodiment shown in FIG. 25, the seat 26″ has a lower taperedportion 26 a″ that tapers and narrows towards the first or distal end 21of the holding member 2″. The seat 26″ also includes a cylindricalportion 26 b″ in a direction towards the second end 22 of the holdingmember 2″. The inner diameter of the cylindrical portion 26 b″ isslightly larger than the largest diameter of the head 102. When the arms25 a, 25 b are pressed together, the tapered portion 26 a″ exerts aforce on the head 102 that urges or tries to move the head 102 upwardtowards the second end 22 of the holding member 2″. Because of theabutment of the head 102 against the abutment surface 14 a, the head 102is firmly pressed against the abutment surface 14 a.

In the embodiment shown in FIG. 26, the seat 26′″ includes a firsttapered portion 26 a′″ that is tapered and narrows towards the first end21 of the holding member 2′″, followed by a cylindrical portion 26 b′″,which is followed by a second tapered portion 26 c′″ that tapers andnarrows towards the second end 22 of the holding member 2′″. Thefunction of the seat 26 a′″ is the same or similar to the otherembodiments.

In all embodiments, the seat 26, 26′, 26″, 26 a′″ has such a size thatwhen the arms are compressed, the head 102 experiences a force thatmoves or urges the head 102 upward in the direction of the abutmentsurface 14 a, whereby the head 102 is pressed against the abutmentsurface 14 a.

In a further modified embodiment, the actuator can be realized by othermeans that facilitate the displacement of the displacement memberbetween the first and the second configurations.

In some embodiments, the actuator can also be omitted. For example, thedisplacement member may have an internal thread that cooperates with anexternal thread of the holding member to facilitate advancement of thedisplacement member along the holding member.

In addition, the abutment surface need not to be circular orring-shaped. Any abutment surface can be used that provides sufficientarea to generate enough friction to eliminate play between theengagement projection of the drive shaft and the engagement recess ofthe head.

The holding member can have more than two arms, for example, three ormore arms, that can be pressed together in a resilient manner.

Meanwhile, instead of the threaded connections shown in the embodiments,other detachable connections can also be provided.

For the bone anchor, any bone anchor can be used that has a shank and ahead portion. It is not necessary that the bone anchor is a bone screwthat has to be screwed in. For example, the instrument can also besuitable for pushing a bone anchor into a hole.

In addition, while the head of the bone anchor and the seat of theinstrument are shown as being used in conjunction with a polyaxial boneanchor, it is also possible, for example, that the head and the seathave flat side portions that instead render the bone anchor suitable fora monoplanar bone anchoring device that allows pivoting only in a singleplane.

While the present invention has been described in connection withcertain embodiments, it is to be understood that the invention is notlimited to the disclosed embodiments, but is instead intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims, and equivalents thereof.

The invention claimed is:
 1. An instrument configured to hold a boneanchor comprising a shank for anchoring to a bone and a head, theinstrument comprising: a holding member configured to hold the head ofthe bone anchor, wherein a first end of the holding member comprises aplurality of arms configured to at least partially encompass the head,and wherein an inner surface of the arms forms a seat for the head; adrive shaft defining a longitudinal axis of the instrument, the driveshaft comprising an engagement portion for engaging the head of the boneanchor; and a displacement member; wherein when the holding member, thedrive shaft, and the displacement member are assembled to one another,the holding member is positionable around and movable over a majority ofthe drive shaft, while the drive shaft is configured to directly engageanother portion of the instrument to keep the drive shaft recessed andspaced apart axially from the first end of the holding member, and theinstrument is adjustable from a first configuration wherein the arms ofthe holding member are movable radially outwardly to permit the head ofthe bone anchor to enter the seat, to a second configuration wherein thedisplacement member restricts the arms from moving radially outwardly tohold the head in the seat.
 2. The instrument of claim 1, wherein theseat comprises a spherical inner surface portion.
 3. The instrument ofclaim 2, wherein a region of a largest inner diameter of the sphericalinner surface portion of the seat is situated between an edge of theseat closer to the first end of the holding member and an edge of theseat farther away from the first end of the holding member.
 4. Theinstrument of claim 1, wherein the drive shaft has a free end thatcomprises the engagement portion, and wherein the drive shaft furthercomprises an abutment surface provided at a distance from the free endfor abutting against an end of the head.
 5. The instrument of claim 1,wherein the holding member comprises a first sleeve with at least oneslot extending from the first end, such that the arms are formed by theat least one slot and are resilient towards each other.
 6. Theinstrument of claim 5, wherein the drive shaft is configured to extendthrough the sleeve and to connect to a second end of the holding memberopposite the first end.
 7. The instrument of claim 1, wherein thedisplacement member comprises a sleeve positionable around at least partof the holding member and movable axially relative to the holdingmember.
 8. The instrument of claim 7, further comprising an actuator foractuating the displacement member and for holding the instrument at thefirst configuration and at the second configuration.
 9. The instrumentof claim 8, wherein the actuator comprises another sleeve configured tobe connected to the drive shaft via a threaded connection.
 10. Theinstrument of claim 8, wherein the displacement member is connectable tothe actuator such that a movement of the actuator in the longitudinaldirection displaces the displacement member in the longitudinaldirection.
 11. The instrument of claim 1, wherein the holding member isdetachably connected to the drive shaft.
 12. The instrument of claim 8,wherein the actuator is detachably connected to the drive shaft.
 13. Asystem comprising the instrument of claim 2 and the bone anchorcomprising the shank and the head.
 14. The system of claim 13, whereinthe head of the bone anchor has a spherical outer surface portion. 15.The system of claim 14, wherein when the head is held in the seat andwhen the instrument is at the second configuration, a center of a spheredefined by the spherical outer surface portion of the head is positionedaxially between the first end of the holding member and a center of asphere defined by the spherical inner surface portion of the seat. 16.The instrument of claim 9, wherein the threaded connection between theactuator and the drive shaft comprises a multi-start thread.
 17. Theinstrument of claim 8, wherein the actuator is detachably connected tothe displacement member.
 18. The instrument of claim 1, wherein the seatcomprises a first tapered inner surface portion that narrows towards thefirst end of the holding member.
 19. The instrument of claim 18, whereinthe seat further comprises a second tapered inner surface portion on aside of the first tapered inner surface portion opposite the first endof the holding member, and wherein the second tapered inner surfaceportion widens towards the first end of the holding member.
 20. Aninstrument configured to hold a bone anchor comprising a shank foranchoring to a bone and a head, the instrument comprising: a holdingmember configured to hold the head of the bone anchor, the holdingmember comprising a plurality of arms configured to at least partiallyencompass the head, wherein an inner surface of the arms forms a seatfor the head; a drive shaft defining a longitudinal axis of theinstrument, wherein the drive shaft is positionable at least partiallythrough the holding member and comprises an engagement portion forengaging the head of the bone anchor; and a displacement memberpositionable around at least part of the holding member; wherein whenthe holding member, the drive shaft, and the displacement member areassembled to one another, the drive shaft directly engages both theholding member and the displacement member, and the displacement memberis movable axially relative to the drive shaft and relative to theholding member from a first configuration wherein the arms of theholding member are movable radially outwardly to permit the head of thebone anchor to enter the seat, to a second configuration wherein thedisplacement member restricts the arms from moving radially outwardly tohold the head in the seat.
 21. An instrument configured to hold a boneanchor comprising a shank for anchoring to a bone and a head, theinstrument comprising: a holding member configured to hold the head ofthe bone anchor, wherein a first end of the holding member comprises aplurality of arms configured to at least partially encompass the head,and wherein an inner surface of the arms forms a seat for the head; adrive shaft defining a longitudinal axis of the instrument, wherein thedrive shaft is positionable at least partially through the holdingmember and comprises an engagement portion for engaging the head of thebone anchor; and a displacement member positionable around at least partof the holding member; wherein when the holding member, the drive shaft,and the displacement member are assembled to one another, the driveshaft is configured to directly engage another portion of the instrumentto keep the drive shaft recessed and spaced apart axially from the firstend of the holding member, and the displacement member is movableaxially relative to the drive shaft and relative to the holding memberfrom a first configuration wherein the arms of the holding member aremovable radially outwardly to permit the head of the bone anchor toenter the seat, to a second configuration wherein the displacementmember restricts the arms from moving radially outwardly to hold thehead in the seat.